Self-curable or self-hardenable (hereafter "self-curable") resinous or elastomeric material such as is used for sealants, adhesives and caulking of an article, which is e.g. to be bonded to another by means of the adhesive, is well known in the prior art. By "self-curable" in its strictest sense is meant that the material is cured fully, after having been applied, under the influence of environmental conditions such as moisture available from humid air, or by oxygen in the atmosphere, or by light energy, especially in the ultraviolet range, or by thermal energy, that is, under the influence of elevated temperature, particularly in the absence of auxiliary curing agents to be admixed to the material. The rate of cure as a function of time depends upon the amount of moisture or oxygen, or the amount of thermal or light energy available from the environment. Raising the temperature to accelerate the cure does not require the addition of another accelerator in the self-curable material, but relatively few thermally accelerated materials are reliably controllable. Anyway the curing process may take a considerable period of time, typically few hours or even several days.
A curable material in the sense of this invention includes a self-curable material which upon adding and admixing an additional curing agent in a sufficient amount will cure fully and quickly, such cure requiring only a few minutes or seconds.
For example, a polyester may be mixed with a small amount of an organic peroxide catalyst sufficient to be slowly self-curing at ambient temperature, but benefit from an additional amount of the same or another catalyst and/or accelerator to accelerate the cure. For such self-curable materials, as well as those (such as polyurethanes) which require no curing agent mixed therein (because they are moisture-curable), an accelerator is desirably incorporated just prior to application of the mixture to the article. As will be explained hereinafter in greater detail, the use of a mixture of at least two components, one of which is self-curable, and which mixture is quick-curing, is generally impractical at the present time in production manufacturing equipment, because of the detrimental consequences of having such a mixture cure in the equipment and lines used to apply the mixture.
To add an accelerator separately to accelerate the cure of a self-curable material is not obvious, since the material is already provided with a preselected rate of cure, and therefore would not require an added accelerator. Conventional two-component systems are created to ensure a quick cure in the range from a few (about 10 to 50) seconds to a few (about 1 to 10) minutes, between components which will not cure without being reacted. Even such systems are difficult to use because a mixture of the two components must be used continuously, without an interruption long enough to permit the mixture to cure in the equipment. The difficulty is exacerbated when one component is self-curable without interaction with another, and an accelerator is mixed into the material to accelerate its cure within less than a minute or a few minutes.
The present invention is limited to multi-component systems of the type in which at least one component is self-curable, and at least one other component accelerates the cure of the self-curable material. All references hereafter to a "two-component system" refer to a multi-component system in which at least one component of the mixture to be applied is a self-curable resin or elastomer, and at least one other component is an accelerator. The term "resin" is used to define an organic material which hardens to a rigid form having little, if any, elongation before break at room temperature. The term "elastomer" is used to describe a noticeably flexible, or rubbery material having substantial elongation before break even if it is tough.
To apply such a mixture essentially continuously to an article, the components which are kept separately in storage vessels or containers, are desirably pumped through metering pumps and mixed continuously while flowing through a conduit from which the mixture is applied to the article, coating it as desired. It will be understood that prolonged storage of each of the components is practical, typically in containers into which neither air nor light is admitted, as long as the contents of the containers are kept separate. As soon as the contents are mixed, the mixture begins to cure.
In a continuous operation during which the two components are mixed, and the mixture is uninterruptedly applied to a rapid succession of articles in successive coating operations, quickcuring of the self-curable material to its predetermined substantially fully cured conditions is highly desirable. But, if there is a significant interruption in the application of the mixture, it will cure and plug whichever portion of the equipment in which the mixture remains.
When such an interruption is expected, the mixture may be washed out of the lines and such mixing means and applicator conduit as may be used, with a suitable solvent, until not enough mixture is left in the equipment to cause a problem starting up again. To be sure, interruptions are undesirable but they are unavoidable in many circumstances such as those occasioned by a change in shift or other pauses during working hours, or by an unforeseen disturbance or breakdown of an operation in the overall production system.
The disadvantage of this conventional procedure for immediately cleaning out the equipment is that the solvent used is a wasted expenditure. Moreover, environmental considerations necessitate expensive recovery units, the operation of which, in turn, consumes much energy. A further disadvantage is that a significant loss of valuable material is incurred because it is washed away. They are such disadvantages which are detrimental to profitable operation and should be avoided.